Self-locking threaded fastening assembly

ABSTRACT

A bolt ( 10 ) with a partially threaded shank has a plurality of locking grooves located on the shank between the thread and its head ( 11 ). The bolt ( 10 ) is inserted through a component stack and spacing washers and a stepped reaction washer such that at least half of the locking grooves ( 13 ) are left exposed. A deformable frusto-conical locking ring ( 17 ) is positioned on a nut ( 15 ) having a circular receiving cavity on one end. The nut ( 15 ) and locking ring ( 17 ) are positioned on the thread ( 14 ) of the bolt ( 10 ) and tightened so that the locking ring ( 17 ) is deformed from a frusto-conical shape to a substantially flat disc such that its bore diameter is reduced, thereby retaining it within one of the locking grooves ( 13 ). The outer diameter of the receiving cavity ( 16 ) abuts the stepped reaction washer ( 18 ).

[0001] The present invention relates to a self-locking threadedfastening assembly.

[0002] Conventional threaded fasteners may comprise a nut and acorresponding threaded portion on e.g a bolt or a threaded stud whichcan be used to fasten components. The nut may be locked on the bolt bymeans of a split pin. If further security is required, one earlier formof fastening includes a sprung locking ring provided on the bolt, whichis permanently located in a groove between the thread and a plainportion of the shank of the bolt. On fitment, the split sprung ringdepresses so that it sits flush with the shank diameter whilst the boltis pushed through the bolthole until it appears on the opposite side.Once the bolt has been inserted, the split sprung ring opens out,preventing the bolt from being retracted through vibration or rotationalmechanical actions. A nut is then fitted onto the bolt and where yetmore security is required, e.g. for flying controls of an aircraft etc,a split pin would also be fitted. Even if the nut and split pin fail inservice, the locking function exerted by the spring locking ringremains.

[0003] In practice the component stack may be too wide due to a build upof tolerances, resulting in a need for a bolt of alternative length.This replacement can be costly and time consuming as several componentsmay need to be stripped down and rebuilt. Allowing for the difference inwidths of component stack and length of the bolt can also haverepercussions for other aspects of the design.

[0004] U.S. Pat. No. 5,393,183 shows an assembly having a captive nutand an internally threaded nut but is non-adjustable and requiresvarious bolt lengths in order to satisfy various stack situations. U.S.Pat. No. 3,638,980 shows a self-retained bolt assembly but is alsonon-adjustable and ones of different sizes would have to be manufacturedto deal with different size component stacks.

[0005] GB 2340909 discloses a nut and bolt locking system having awasher made of a resilient material. However, the spring lock can bemanually pushed back to allow the nut to be removed, which means that itcould come loose or be taken apart by unauthorised personnel. GB 1006102discloses a locking device with a bolt having a helical thread, whichcan still result in similar problems to those described for the otherknown devices.

[0006] An object of the present invention is to provide an alternativeform of self-locking fastening assembly, and preferably one which allowsfixing of component stacks of variable width. In a preferred embodiment,the assembly and removal of the fixing means can be achieved usingexisting tools.

[0007] According to a first aspect of the present invention there isprovided a self-locking fastening assembly including:

[0008] a male member having a shank, the shank including at one endregion a helical threaded portion and further including a plurality ofnon-helical locking grooves located on the shank adjacent or inboard ofthe threaded portion;

[0009] a locking ring which in use is deformed on compression thereof sothat its inner diameter contracts such that it can locate in and beretained by one or more of said locking grooves; and

[0010] a nut for engaging said threaded portion and applying compressionto said locking ring.

[0011] The provision of a plurality of locking grooves means that thelocking device may be used with stacked components where the stackdimension varies, whilst still providing a good “tight” connection.

[0012] Preferably, the locking ring is of substantially dished orfrusto-conical shape.

[0013] The fastening assembly may further include a reaction washer, thereaction washer being located in use on the shank adjacent the lockingring. The reaction washer is preferably at least partially tapered orfrusto-conical in shape, and in use the inner diameter of the reactionwasher preferably abuts the inner diameter of the locking ring.

[0014] Preferably, the nut has a receiving cavity at one end defined byan outer circumferential lip, the receiving cavity having suchdimensions that the locking ring can be at least partially containedwithin it. Normally, when the fastening assembly is assembled, the outerdiameter of the locking ring abuts the receiving cavity. In use, the lipof the receiving cavity preferably abuts the reaction washer, therebyallowing the clamping force exerted by the nut to be transmitted to thewasher, and not just the locking ring, when the nut is tightened.

[0015] The fastening assembly may further include at least one spacingwasher arranged in use inboard of the reaction washer.

[0016] The male member may be a bolt or a threaded stud.

[0017] Whilst the invention has been described above, it extends to anyinventive combination of the features set out above or in the followingdescription.

[0018] The invention may be performed in various ways, and, by way ofexample only, an embodiment thereof will now be described by way ofexample only, reference being made to the accompanying drawings, inwhich:

[0019]FIG. 1 illustrates the components of a preferred embodiment of aself-locking bolt assembly before being assembled,

[0020]FIG. 2 illustrates the components of FIG. 1 in an assembledconfiguration fixing a component stack, and

[0021]FIG. 3 illustrates a detailed view of portion A of FIG. 2.

[0022] The Figures illustrates a self-locking fastening in the form of aself-locking bolt.

[0023] In FIG. 1 a bolt 10 has a head 11 at one end with a cylindricalshank. A portion 12 of the shank adjacent to the head 11 has asubstantially smooth, plain surface. A portion of the shank outboard ofthe plain portion 12 and remote from the head 11 includes a plurality ofannular non-helical locking grooves 13. The locking grooves 13 areformed on the shank by using a threading tool, which may be a threadcutting or thread rolling tool, without any traverse for producing ahelical pitch. As standard threading tools can be used, the bolt 10 mayhave an initially smooth shank, or a standard bolt having a helicalthread can be adapted to include locking grooves. Thus, if no smoothshanked bolts of suitable size are available then standard bolts can beused.

[0024] A further portion of the shank, adjacent the locking grooves 13and remote from the plain portion 12, includes a helical thread 14. Thenon-helical locking grooves 13 are similar in depth and shape to thethread 14, and thus the shear strength of the bolt is not reduced.

[0025] The assembly also includes a nut 15. The nut 15 has a threadedbore of diameter such that it can be threaded onto the thread 14 of thenut. One end of the nut 15 has a substantially circular receiving cavity16 defined by an outer circumferential lip. The receiving cavity 16 isconcentric with the bore of the nut 15.

[0026] The assembly also includes a locking ring 17. The locking ring 17is substantially frusto-conical in shape and, before being fitted ontothe bolt (as described below), has a usual diameter substantially equalto that of the shank of the bolt to allow it to be slid over thethreaded portion 14. The locking ring may be manufactured from a varietyof malleable or softer materials than the bolt 10 and nut 15. In thepreferred embodiment the locking ring 17 is formed of Monel. In use, thevarious components will be assembled such that the outer diameter of thelocking ring 17 abuts the receiving cavity 16 of the nut 15.

[0027] To the other side of the locking ring 17 from the nut 15 areaction washer 18 faces the locking ring 17. The reaction washer 18 isan annular stepped washer having a frusto-conical surface facing thelocking ring 17.

[0028] The fastening assembly can also include one or more washers 19.The washers 19 are selected by the assembler of the components asrequired, and may be of graded thickness to take up the clearance, e.g.2.54 mm, 1.27 mm, etc.

[0029] Referring now to FIG. 2, the self-locking bolt is shown clampinga stack 21 of components. To do this the bolt 10 is inserted through thecomponent stack 21 such that the locking grooves 13 of the bolt protrudeout through the component stack. The washers 19 and reaction washer 18are then fitted onto the bolt, such that the washers 19 abut thecomponent stack 21 and about half of the locking grooves 13 are leftprotruding. The locking ring 17 and the nut 15 is positioned on the freeend of the bolt 10 and the nut are tightened by torque loading. The nut15 is then locked in place using a split pin 20 in the usual manner.

[0030] As can be seen in FIG. 3, as the nut 15 is tightened to thecorrect torque, the frusto-conical surface of reaction washer 18 causesthe frusto-conical locking ring 17 to be deformed to a substantiallyflat form, so that its bore diameter has been reduced, thereby causingit to be seated in one of the locking grooves 13. The lip of thereceiving cavity abuts the outer edge of the reaction washer 18, withthe frusto-conical portion of the reaction washer fitting inside thereceiving cavity. This allows the clamping force to be transmitted fromthe nut via the reaction washer through the washers 19 and componentstack 21 to the bolt head. This feature is also intended to prevent thelocking ring 17 from being deformed beyond a flat shape.

[0031] Due to the presence of the plurality of locking grooves 13 on thebolt shank, it is possible to fit the bolts to assemblies of varyingstack width, typically by up to 5 mm, thus reducing the delay inprocuring an alternative bolt length or a costly strip down of thestructure.

[0032] To remove the fastening assembly to allow the component stack tobe disassembled, the nut 15 is unthreaded off the bolt 10 and the boltis tapped until the locking ring 17 is free. This is similar to themanner in which a conventional self-locking bolt with a sprung lockingring is removed, and so the illustrated embodiment does not require anyspecialised equipment in order to be assembled and disassembled.

[0033] This fastening assembly may be used on many engineering productsand can act as a “tamper proof” device to deter unauthorised stripdowns. Also, on products which do not require more than two lockingdevices, the illustrated embodiment can be used on its own in thosedifficult places instead of split pins.

[0034] Thus, the preferred embodiment provides a secure lockingmechanism which can be used in addition to conventional locking devicessuch as a nut and split pin. The availability of an additional lockingmechanism is particularly useful in aircraft manufacture, as aviationauthorities provide guidelines for the minimum number of locking deviceswhich are to be used for certain components, e.g. for flying controls aminimum of three locking devices are stipulated, which could beimplemented using a nut, a split pin and the locking ring of thepreferred embodiment.

[0035] A fourth locking mechanism could be added by using a lock nut,which may be of the nylon or compressed thread type (i.e. one in whichon tightening the thread in the nut is compressed) instead of a standardnut, thus giving four locking devices: the nut, the compressed threadwithin the nut, the split pin and the locking ring of the preferredembodiment. In other cases, the designer or engineer may stipulate twolocking devices, which would conventionally be implemented by means of anut and a split pin. However, where sufficient space is unavailable oraccess is difficult, the split pin may be replaced by a locking nut orthe locking ring of the preferred embodiment which provides asubstantially tamper-proof locking arrangement.

1. A self-locking fastening assembly including: a male member having ashank including at one end region a helical threaded portion and furtherincluding a plurality of non-helical locking grooves located on theshank adjacent or inboard of the threaded portion; a locking ring whichin use is deformed on compression thereof so that its inner diametercontracts such that it can be located in and retained by one or more ofsaid locking grooves, and a nut for engaging said threaded portion andapplying compression to said locking ring.
 2. A self-locking fasteningassembly according to claim 1, wherein the locking ring is ofsubstantially dished or frustro-conical shape.
 3. A self-lockingfastening assembly according to claim 1 or claim 2, further including areaction washer, the reaction washer being located in use on the shankadjacent to the locking ring.
 4. A self-locking fastening assemblyaccording to claim 3, wherein the reaction washer is at least partiallytapered or frusto-conical in shape.
 5. A self-locking fastening assemblyaccording to any one of the preceding claims, wherein the nut has areceiving cavity at one end defined by an outer circumferential lip, thereceiving cavity having such dimensions that the locking ring can be atleast partially contained within it.
 6. A self-locking fasteningassembly according to claim 5, when dependent upon claim 3 or claim 4,where in use the lip of the receiving cavity abuts the reaction washer.7. A self-locking fastening assembly according to any one of claims 3 to6, further including at least one spacing washer, the spacing washerbeing located in use adjacent the reaction washer.
 8. A self-lockingfastening assembly according to any one of the preceding claims, whereinthe male member is a bolt.
 9. A self-locking fastening assemblyaccording to any one of the preceding claims, wherein the male member isa threaded stud.